The present invention relates to downhole drilling systems.
In general, there are two approaches for carrying out the rotation of the drill during a drilling operation. Either the entire drill string on which the drilling bit is mounted can be rotated by a rotary mechanism from the surface, e.g. by a rotary table, or alternatively, the rotation of the drill bit can be carried out by a downhole motor. Typically, when rotating the pipe sections, a drilling fluid is pumped down through the drill pipe to help flush out the rock cuttings. The drilling fluid is then returned up the space between the drill string and the side of the hole being drilled. In general, the drilling fluid is a mud mixture which also serves to cool and lubricate the drill bit as the cuttings are removed. In order to enable the drill string with the attached bit to be rotated, mounted at the upper end of the drill string is a kelly. While the kelly engages the cylindrical drill pipe of the drill string, its external surface is non-cylindrical, typically hexagonal. The kelly then passes through an opening in the rotary table which has a corresponding shape as the external circumferential surface of the kelly. Thus the rotary table engages the kelly and by spinning the table, the kelly and correspondingly the drill string and the drill bit are rotated. As the drill string advances forward in the earth, the drilling operation needs to be periodically stopped so as to add an additional pipe section. When employing a rotary table along with the hose assembly for supplying the drilling fluid, this procedure for adding an additional pipe section becomes a complex and a time consuming operation.
Under certain circumstances, it becomes desirable to utilize a downhole drilling motor in place of the rotary table. The deeper the hole, the further the drilling bit from the rotary table that supplies the power from the earth's surface. Thus, there is a great energy loss between the power source and the drilling bit. In such a situation, it is preferable to place the power source as close to the drilling bit as possible, thus, it becomes beneficial to employ a downhole motor. While the motor rotates the bit, a major factor in accomplishing the forward advance of the drilling bit is the weight applied by the drill string. Thus, the downhole drilling system must be capable of transmitting radial forces which are at an angle to the longitudinal axis of the drill string and longitudinal forces which are generally parallel to the longitudinal axis of the drill string. One exemplary embodiment of such a downhole drilling system is illustrated and described in U.S. Pat. No. 3,730,284 to Striegler. Other embodiments of downhole drilling systems employing various kelly bushings, are illustrated and described in U.S. Pat. Nos. 3,842,619 to Bychurch; 3,854,539 to Sweeney; and 3,913,352 to Oliver.
During many points of the drilling operation, it is desirable to place some type of sensing equipment in the area of the downhole motor for feeding back to the surface various information concerning both the drilling operation and the characteristics of the hole being drilled. When employing such sensing equipment, an electric line from the surface must be connected to the sensing equipment. Prior to the present invention, when utilizing such sensing equipment, it has been necessary to employ a conventional type drilling system of a rotary table along with the necessary hoses for supplying the drilling fluid. With previously known arrangements for downhole drilling, there has not been any capability of feeding an electric line into the hole to the sensing equipment. Thus, it has not been possible to fully exploit the advantages of a downhole drilling system in all situations because of such limitations. By requiring the use of the rotary table and hoses for the drilling fluid whenever a sensing mechanism is to be employed, due to the extra time involved in adding additional pipe sections to the drill string, the complexity of the drilling operation and time involved is significantly increased.